1. Field of the Invention
The invention relates to air-operated diaphragm brakes for vehicles and, more particularly, to tandem service and emergency brake actuator assemblies having increased braking output and decreased hold off pressure requirements in the emergency brake.
2. State of the Prior Art
An air brake system for a vehicle such as a bus, truck or the like typically includes a brake shoe and drum assembly which is actuated by means of an actuator assembly operated by the selective application of compressed air. Conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and an emergency brake actuator which causes actuation of the brakes when air pressure has been released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This is often referred to as the spring brake actuator. Typically, the spring brake actuator is disposed in tandem with the service brake actuator. When full pressure is applied to the emergency brake actuator, air pressure acting against a diaphragm compresses the spring. A spring brake actuator rod is held in a retracted position by a relatively small return spring, thus not affecting the operation of the brake. When the brake is to be applied during normal driving operation, compressed air is provided to the service brake actuator which, acting against a diaphragm, causes a service brake push rod to be extended and causes the brakes to be applied with an application force which is proportional to the air pressure applied to the service brake actuator. In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator, the brake will be mechanically activated by the force of the emergency brake spring acting on the spring brake actuator rod which in turn acts upon the service brake push rod to apply the brakes. Thus, the spring brake portion serves both as a parking brake and an emergency brake.
A typical brake actuator has a certain range of braking effectiveness, which is defined by the effective stroke length of the service brake push rod. This braking effectiveness is reduced as the push rod moves beyond its effective stroke length and approaches its limit of travel. As the service brake push rod reaches its limit of travel when the brakes are applied, the force at which the push rod moves outwardly is reduced since the effective area over which the air pressure acts is reduced in the service brake housing.
A typical actuator may have an effective area of 30 square inches in the service brake housing at the beginning of the rod stroke. This effective area is directly related to the size of the diaphragm and the size of the pressure plate adjacent to the diaphragm in the service brake housing. A constant air pressure acts across the entire diaphragm, including the portion not supported by the pressure plate. The service brake push rod itself is designed typically to have a working stroke length of 0 to 21/2 inches, where 0 to 11/2 or 2 inches is the effective stroke length. As the rod is extended through the effective stroke length, the effective area may decrease slightly or remain constant. As long as the effective area remains constant, the force on the rod to actuate the brakes also remains constant, since the force is equal to the effective area multiplied by the constant air pressure. At the end of its effective stroke length, the diaphragm becomes rounded at the pressure plate, thus reducing the effective area against which the air pressure acts. The lower portion of the housing itself may be canted inward, thus restricting diaphragm movement and leading to an even greater reduction in the effective area. Consequently, as the rod passes from the 11/2 or 2 inch stroke to its fully extended position at 21/2 inches, the braking force decreases.
In one prior art actuator, the effective area can be reduced to as little as 22 square inches. For a typical air pressure of 15 psi when the brakes are applied, the brake force ranges from 450 lbf (pounds-force) over the effective stroke length with an effective area of 30 square inches, to 330 lbf beyond the effective stroke length, when the effective area is reduced to 22 square inches. This sharp reduction in braking force ordinarily does not pose a problem when the rod is within its effective stroke length. However, when the brakes are at the end of their service life or improperly maintained or adjusted, the rod must travel beyond its effective stroke length to apply the brakes. This pertains as well when the brakes become overheated due to constant use, such as when descending long grades. The brake drums may heat up and expand due to friction between the brake shoes and drums. Consequently, a longer stroke is required to apply the brakes. The operator typically pushes harder on the pedal expecting the braking effectiveness or the force on the brakes to increase. However, just the opposite happens. As the rod passes its effective stroke length, the braking effectiveness actually decreases. Thus, less braking action is accomplished as the operator pushes further on the pedal.
Another problem associated with the air brake actuator occurs in the spring brake housing. Typically, the spring brake housing has an actuator rod aligned with the push rod in the service brake housing. One end of the actuator rod terminates in a reaction plate which abuts against the push rod. The other end of the actuator rod terminates in a pressure plate which engages a spring brake compression spring. A diaphragm abuts the pressure plate and sealingly divides the spring brake housing into an upper spring chamber and lower air chamber. The air chamber is filled with compressed air supplied through an air service port when the emergency brake is in its normally released position. Although the required amount of air pressure may vary depending on the particular design, a pressure such as 100 psi is typical. As in the service brake chamber, the diaphragm and pressure plate in the spring brake chamber define an effective area against which the air pressure acts, i.e., a typical effective area could be 30 square inches, which, together with a constant pressure of 100 psi, would result in a force of 3,000 lbf to keep the compression spring retracted. When a leak occurs in the air pressure system, it is difficult to maintain the required constant pressure.
Consequently, the compression spring advances against the lower air pressure until an equilibrium state is reached between the spring and the air chamber. The actuator rod is simultaneously pressed against the push rod in the service brake housing, and may extend the push rod far enough to cause the brake shoes to drag against the drums, a situation with potentially serious consequences. At the very least, the brake shoes and drums are subject to faster wear, leading to shorter service intervals and increased costs due to parts, labor, and down time. Furthermore, the high pressure that must be maintained in the spring brake housing results in large air reservoirs and air compressors, which contribute to increased overall cost of the braking system.